This invention relates generally to methods for computed tomographic (CT) imaging, and more specifically to methods and apparatus for rapidly reconstructing helically scanned images.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array f radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. In a helical scan, a table supporting the object moves through the gantry as it scans.
In a multislice imaging system, the detector comprises a plurality of parallel detector rows. A multislice detector is capable of providing a plurality of images representative of a volume of an object. Each image of the plurality of images corresponds to a separate xe2x80x9cslicexe2x80x9d of the volume. The thickness or aperture of the slice is dependent upon the thickness of the detector rows. It is also known to selectively combine data from a plurality of adjacent detector rows (i.e., a xe2x80x9cmacro rowxe2x80x9d) to obtain images representative of slices of different selected thicknesses.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display. Imaging programs of one known CT imaging system rely on a 2-dimensional (2D) backprojection. Accordingly, cone angles of the individual line integrals are ignored and all the rays acquired at a given source position and at a given detector row are described as belonging to a single plane orthogonal to the z axis. This plane is uniquely described by its z-distance to the gantry plane. The xe2x80x9cgantry planexe2x80x9d is a plane orthogonal to a z-axis (or patient axis) and passing through the center of the focal spot. In general, the gantry plane exactly bisects the detector in z, that is, it passes between two rows of the detector. Due to the fact that a 2D backprojection is used for reconstruction, all fan rays measured by a detector macro-row (for a given source position) are assumed to be coplanar, in a plane orthogonal to z. The associated plane of reconstruction (POR) is uniquely characterized by its distance to the gantry plane, which distance depends upon a selected imaging aperture. The POR intersects the z-axis where the center (in z) of the associated macro-row projects.
In a xe2x80x9chigh speedxe2x80x9d (HS) mode of a CT imaging system, the width of the detector crosses a POR of an image in a fraction of a full rotation. Specifically, in one known CT imagining system, this fraction is {fraction (4/6)} (=0.67) for a four-slice scanner at 6:1 pitch, from detector edge to detector edge (allowing for some data extrapolation, over half a macro-row); {fraction (3/6)} (=0.5) for a four slice scanner at 6:1 pitch, without data extrapolation; {fraction (8/11)} (=0.73) for an eight slice scanner at 11:1 pitch, from detector edge to detector edge; and {fraction (7/11)} (=0.64) for an eight slice scanner at 11:1 pitch, without data extrapolation. These fractions are to be compared with the fraction necessary for half-scan reconstruction, which is (xcfx80+2xcex93)/(2xcfx80)=0.65, where xcex93 is the maximum fan angle.
Therefore, it is seen that known HS modes barely provide enough data for half-scan reconstruction (that is, they provide just enough data when allowing for a small amount of extrapolation from the detector rows). It is also seen that at a pitch of 11:1 for one known eight slice system, just enough data is provided for image reconstruction from a half-scan data acquisition.
Known helical weighting algorithms require a large number of super-views, i.e., sets of projection data acquired at a given view angle, with as many projections as detector rows. Based on the considerations above, it would be desirable to provide methods and apparatus for reconstruction of images that require a reduced number of super-views and reduced processing time. It would further be desirable to provide methods and apparatus for reconstruction of images that provide improved temporal resolution.
In one embodiment, the present invention is a method for reconstructing a computed tomographic image of an object that includes steps of: helically scanning an object with a multislice CT imaging system to collect projection data; identifying a center view of the collected projection data; performing an operation selected from the group of helical interpolation and helical extrapolation on the collected projection data to produce super-views of the object; weighting an angular range of the super-views with normalized helical weights, the normalized helical weights being dependent upon whether interpolation or extrapolation was performed; and backprojecting the weighted super-views to produce a reconstructed image of the object.
This embodiment provides an advantage of requiring a reduced number of super-views and reduced processing time compared to known image reconstruction methods. In addition, improved temporal resolution relative to known methods is also provided.